The majority of new HIV-1 infections in the world are acquired by women after heterosexual contact. During sexual transmission, HIV-1 must get across the genital mucosa, infect and replicate in the early target cells, and then disseminate from the initial site of invasion to generate a systemic infection. HIV-1 primarily replicates in CD4+ lymphocytes, but these cells are mostly absent from intact mucosal surfaces. Tissue based dendritic cells (DCs), known as Langerhans cells (LCs), are generally believed to facilitate HIV-1 access to CD4+ T cells. Because it has been difficult to isolate tissue based dendritic cells, most studies employ monocyte derived dendritic cells (MDDCs) or skin derived LC as surrogates. We have developed novel methods to isolate tissue based dendritic cells termed CD1a+ vaginal dendritic cells (VDCs) from discarded vaginal tissue. The previously uncharacterized CD1a+ VDCs possess unique cell surface receptors, intracellular features and susceptibility. Most importantly, we show that HIV-1, able to utilize the CCR5 receptor, (termed R5) is able to replicate in CD1a+ VDCs while variants that use the CXCR4 receptor (termed X4) fail to infect these cells. Because viruses acquired by newly infected subjects predominantly use the CCR5 receptor although transmitting partners often harbor both R5 and CXCR4 utilizing virions, our data explains this most fundamental observation about coreceptor restriction during HIV-1 acquisition. We hypothesize that CD1a+ VDC project dendrites into the vaginal lumen. These dendrites allow CCR5 but not CXCR4 using viruses to be acquired from the vaginal lumen. Local replication in CD1a+ VDCs facilitates virus transfer across the mucosal epithelium to deeper lying CD4+ T cells. Besides increasing HIV-1's ability to infect and replicate in deeper lying CD4+ lymphocytes, we hypothesize that CD1a+ VDCs also determine which HIV-1 variants establish a productive infection within a naive host. During sexual transmission, a newly infected subject acquires only a limited number of R5 variants from the diverse X4 and R5 viruses circulating in the chronically infected transmitting partner. In addition, infecting R5 viruses often have unique genotypes and phenotypes compared to the quasispecies present in the transmitting partner. We hypothesize that CD1a+ VDCs function as gate keepers, both blocking X4 viruses and favoring the replication of R5 variants with properties often found among HIV-1 isolated early after infection. In the proposal, we will examine mechanisms that allow CD1a+ VDC to be susceptible to R5 but resistant to X4 HIV-1. We will examine if these cells allow preferential replication of the infecting strains compared to non-transmitted viruses. We will also explore inhibitors that can prevent replication in CD1a+ VDCs and transfer of virus to CD4+ T cells. Delineating the mechanistic details and factors that can prevent infection in CD1a+ VDCs or virus transfer to CD4+ T cells will suggest novel vaccine and microbicide strategies aimed at preventing HIV-1 mucosal transmission.